In the field of so-called “thermal” infrared detectors, it is known to use one-dimensional or two-dimensional arrays of elements sensitive to infrared radiation, capable of operating at ambient temperature.
A thermal infrared detector conventionally uses the variation of a physical quantity of a so-called “thermometric” or “bolometric” material, according to its temperature. Most currently, this physical quantity is the electric resistivity of said material, which is strongly temperature-dependent. The unit sensitive elements of the detector, or “bolometers”, are usually in the form of membranes, each comprising a layer of thermometric material, and suspended above a substrate, generally made of silicon, via support arms having a high thermal resistance, the array of suspended membranes being usually called “retina”.
Such membranes especially implement an incident radiation absorption function, a function of conversion of the power of the absorbed radiation into heat power, and a thermometric function of conversion of the generated heat power into a variation of the resistivity of the thermometric material, such functions being implementable by one or a plurality of distinct elements.
Further, the support arms of the membranes are also conductive and connected to the thermometric layer of said membranes, and means for sequentially addressing and biasing the thermometric elements of the membranes and means for forming electric signals usable in video formats are usually formed in the substrate having the membranes suspended thereabove. The substrate and the integrated means are commonly called “read circuit”.
The read circuit and the sensitive retina of a detector are usually integrated in a sealed package under very low pressure, provided with a window transparent to the radiation of interest, usually having a wavelength in the range from 8 to 14 micrometers. This range corresponds to the transparency window of the atmosphere and to the majority of the radiations originating from scenes in the vicinity of 300 K. To obtain a thermal or pyrometric image via such a detector, the scene is focused through an adapted optical system onto the focal plane having the retina arranged thereon, and clocked electric stimuli are applied via the read circuit to each of the bolometers, or to each row of such bolometers, to obtain a “video” electric signal forming the image or measurement of the temperature reached by each of said elementary detectors. The signal may be shaped in more or less elaborate fashion, directly by the read circuit, and then transmitted in analog or digital form to an electronic system external to the package. This electronic system typically applies various corrections to each video frame delivered by the detector, in particular a correction of spatial offset and gain dispersions (called “NUC” for Non Uniformity Corrections), to generate a thermal or pyrometric image capable of being displayed, or more generally for the use of the signals thus formed from the observed scene.
Such a detector has many advantages in terms of manufacturing cost and of use, but also disadvantages which may limit its performance in the absence of specific precautions.
More specifically, a problem of drift of the average signal at the output of the bolometers is posed when the substrate temperature varies afterwards, in particular, changes of external conditions, which are essentially reflected by thermal conduction through the integrated system elements all the way to the substrate level, which defines the average temperature of the sensitive membranes. Now, it is well known that the sensitivity of bolometers to a 1° C. variation, for example, of the substrate temperature, is typically from fifty to one hundred times greater than their sensitivity to a 1° C. temperature variation of the observed scene. As a result, in the absence of specific precautions, the useful signal relative to the scene is drowned in this background component deprived of any interest.
To avoid this defect, bolometric detectors operating at ambient temperature have been, since the origin of their industrial development, fitted with a substrate temperature stabilization module, usually a Peltier module (TEC, “Thermo Electric Cooler”). A resistive element regulated by Joule effect in thermal continuity with the substrate also provides a satisfactory temperature stability, although with no active cooling capacity. Such means however make the component more complex and expensive and imply an electric power consumption which is all the higher as the ambient temperature is distant from the selected stabilization temperature. However, the manufacturing cost of such bolometric detectors and the electric power consumption of the system which implements them are precisely the major issue of such detectors.
Actually, the tendency is to do without thermal stabilization systems, and the problem of the adverse effect of substrate temperature variations on the signal stability thus has to be addressed. The solution generally implemented is to arrange, in the electronic circuit for forming the signal in relation with the temperature of the imaging bolometers (called this way since they are sensitive to the incident electromagnetic radiation), an element for compensating the focal plane temperature (FPT), itself bolometric, that is, having its electric behavior following the substrate temperature, but remaining essentially insensitive to radiation. This result is for example obtained by means of bolometric structures provided, by construction, with a lower thermal resistance towards the substrate, and/or by masking these structures behind a shield opaque to thermal radiation.
The use of such compensation elements further has the advantage of eliminating most of the so-called common-mode current originating from imaging (also called “active”) bolometers.
Further, a thermal resolution (smallest temperature gap separable from the background level on the scene) typically smaller than 50 mK (milliKelvin) is required. The laws of radiation and optics provide an attenuation factor of this temperature difference in the range from 50 to 100 between the scene and the membrane of an elementary bolometer. The sensitive elements usually have a temperature coefficient of resistance (TCR) in the order of −2% per degree; these various scale factors result in the need to discriminate a relative variation of resistance smaller than 2·10−5 (20 ppm).
Admitting a resistance read mode at constant bias voltage, minute variations of the current which crosses each bolometric resistor thus have to be discriminated. To achieve this, it is necessary to eliminate most of the so-called common-mode signal independent from the scene, to efficiently depict the spatial and time variations in relation with the observed scene, and this within the available electric dynamic range of the read circuit.
FIG. 1 is an electric diagram of a bolometric detector 10 with no temperature regulation, or “TECless” detector, of the state of the art, comprising a common-mode compensation structure, and FIG. 2 is an electric diagram of a circuit used to form a read signal of a bolometer of the common-mode compensated detector. Such a detector is for example described in document: “Uncooled amorphous silicon technology enhancement for 25 μm pixel pitch achievement”; E. Mottin et al, Infrared Technology and Application XXVIII, SPIE, vol. 4820E.
Detector 10 comprises a two-dimensional array 12 of identical unit bolometric detection elements 14, or “pixels”, each comprising a sensitive resistive bolometer 16 in the form of a membrane suspended above a substrate, such as previously described, and of electric resistance Rac. Each bolometer 16 is connected at one of its terminals to a constant voltage VDET, especially the ground of detector 10, and at its other terminal to a biasing MOS transistor 18 operating in saturated state, for example an NMOS transistor, setting voltage Vac across bolometer 16 by means of a gate control voltage GAC. If A designates the node corresponding to the source of MOS 18 and if VA is the voltage at this node, which depends on gate voltage GAC, voltage Vac is then equal to Vac=VA−VDET. Pixel 14 also comprises a selection switch 20, connected between MOS transistor 18 and a node S provided for each column of array 12, and driven by a control signal Select, enabling to select bolometer 16 for the reading thereof. Transistor 18 and switch 20 are usually formed in the substrate in the footprint of the membrane of bolometer 16. Elements 16 and 18 form a so-called detection branch. Particularly, since the pixels are identical and voltage VDET, on the one hand, and voltage GAC, on the other hand, are identical for all pixels, bolometers 16 are thus voltage-biased under the same voltage Vac. Further, gate voltage GAC being constant, voltage Vac is thus also constant.
Detector 10 also comprises, at the foot of each column of array 12, a compensation structure 22, also usually called “skimming” structure. As previously described, the value of the electric resistance of detection bolometers 16 is greatly dictated by the substrate temperature. The current flowing through a detection bolometer 16 thus comprises a significant component which depends on the substrate temperature and is independent from the observed scene. Compensation structure 22 has the function of delivering an electric current for purposes of partial or total compensation of this component. In the meaning of the invention, the current component of detection bolometers 16 dictated by the substrate temperature is designated as the “common-mode detection current”. The current generated by compensation structure 22 for purposes of compensation of the common-mode detection current is called “common-mode compensation current”.
Structure 22 comprises a compensation bolometer 24, of electric resistance Rcm, made insensitive to the incident radiation originating from the scene to be observed. Bolometer 24 is constructed by means of the same thermometric material as bolometer 16, but has a low thermal resistance towards the substrate. For example, the resistive elements of compensation bolometer 24 are directly formed in contact with the substrate, or bolometer 24 comprises a membrane similar to that of detection bolometers 16 suspended above the substrate by means of structures having a very low thermal resistance, or also compensation bolometer 24 comprises a membrane and support arms substantially identical to those of detection bolometers 16 and a material which is a good thermal conductor fills the space between the membrane of bolometer 24 and the substrate. The electric resistance of bolometer 24 is thus essentially dictated by the substrate temperature, bolometer 24 then being said to be “thermalized” to the substrate.
Bolometer 24 is connected at one of its terminals to a constant voltage VSK, and compensation structure 22 comprises at least one biasing MOS transistor 26 operating in saturated state, of a polarity opposite to that of transistors 18 of detection pixels 14, for example a PMOS transistor, setting voltage Vcm across bolometer 24 by means of a gate control voltage GCM and connected between the other terminal of compensation bolometer 24 and node S. Calling B the node corresponding to the drain of MOS transistor 26 and VB the voltage at this node, voltage Vcm is then equal to Vcm=VSK−VB. Elements 24 and 26 form a so-called compensation branch common to each column.
The value of the common-mode compensation current is defined by the value of resistance Rcm of bolometer 24 and of the biasing parameters thereof.
According to a first variation, resistance Rcm is selected to be substantially identical to that of detection bolometers 16 and the bias voltage of compensation bolometers 24 is selected to be close to that of the detection bolometers, to obtain a common-mode compensation current close to the common-mode detection current. However, it should be noted that it is not necessary to adjust, on design thereof, resistance Rcm of the bolometer 24 to a value close to that of detection bolometers 16, since the current that it conducts should just be adjusted to a value close to that flowing through the detection branch during the reading.
According to a second variation, this result is also obtained by means of a resistance of compensation bolometer 24 smaller than that of detection bolometers 16, and of a bias voltage Vcm smaller by roughly the same proportion.
The second variation is usually preferred since it enables to more efficiently distribute the bias amplitude VSK−VDET available on the bolometric bridge, characteristic of the CMOS technology implemented to manufacture the read circuit. Indeed, it is usually advantageous to further bias the active bolometer to the detriment of the biasing of the compensation bolometer, that is, to impose Vac>Vcm in order to obtain an optimal sensitivity of the detector.
Detector 10 also comprises, at the foot of each column of array 12, an integrator 28 of CTIA type (“Capacitive TransImpedance Amplifier”) for example comprising an operational amplifier 30 and a capacitor 32 connected between the inverting input and the output of amplifier 30. Its inverting terminal and its non-inverting terminal are further respectively connected to node S and to a constant voltage VBUS. Voltage VBUS thus forms a reference for the output signals, and is in the range from VDET to VSK. A switch 34, driven by a signal Reset is also provided in parallel with capacitor 32, for the discharge thereof. The outputs of CTIAs 28 are eventually for example connected to respective sample-and-hold circuits 36 for the delivery of voltages Vout of CTIAs in multiplexed mode by means of a multiplexer 38 towards one or a plurality of series output amplifier(s) 40. It may also be integrated at the output of the digitizing means by analog-to-digital converters (ADC).
Finally, detector 10 comprises a sequencing unit 42 controlling the different previously-described switches. In operation, array 12 is read line by line. To read a line of array 12, switches 20 of the line of pixels 14 are turned on and switches 20 of the other lines are turned off.
After a phase of discharge of the capacitors of the CTIA integrators at the foot of the columns, achieved by closing switches 34 by means of signal Reset, followed by their opening, a circuit such as shown in FIG. 2 is thus obtained for each pixel of the line being read. A current Iac flows through detection bolometer 16 of the pixel under the effect of its voltage biasing by MOS transistor 18, and a current Icm flows through compensation bolometer 24 of the compensation structure under the effect of its voltage biasing by MOS transistor 26. These currents are subtracted from each other at node S, and the resulting current difference is integrated by CTIA 28 during a predetermined integration period Tint. Output voltage Vout of CTIA 28 thus is a measurement of the variation of the resistance of detection bolometer 16 caused by the incident radiation to be detected since the non-useful part of current Iac depending on the substrate temperature is at least partly compensated by current Icm specifically generated to reproduce this non-useful part.
Usually, the bias voltages of detection and compensation bolometers 16 and 24 are adjusted by means of transistors 18 and 26 to position output signal Vout in electric dynamic range Del of integrator 28, at a given reference temperature, usually an average reference temperature with respect to the operational temperature range of the detector, particularly a reference temperature close to 300° K. When the ambient temperature of detector 10 varies, the substrate temperature also varies due to the absence of thermal regulation, and the temperature of detection bolometers 16 thus also varies. However, compensation bolometers 24 being thermalized to the substrate, they are also submitted to a temperature variation resulting from the temperature variation of the substrate. The current flowing through the detection and compensation bolometers thus naturally compensate for each other at the first order when the ambient temperature of the detector, and thus that of the substrate, changes. A substantial rejection of the substrate temperature, also called “focal plane temperature” (FPT), is thus obtained.
If, ideally, a perfect rejection is obtained whatever the substrate temperature, in practice, limitations appear when the variations, particularly positive, relative to the reference temperature exceed a few tens of degrees.
More specifically, bias voltages Vac and Vcm of detection and compensation bolometers 16 and 24 are set so that the average of the output voltages of the detector when the latter is exposed to a uniform scene having a temperature equal to the reference temperature, or “reference scene”, is positioned in the electric dynamic range of integrator 28 at a specific position, considered optimal, of this dynamic range. The output voltages in front of a uniform scene are usually called “continuous levels” and the spatial average of these voltages is usually called “average continuous level”.
When the detector is in service, that is observes a scene of any kind, for a substrate temperature close to the reference temperature, output voltages Vout of the detector exposed to the scene to be observed thus differs from the average continuous level, particularly due to the heating of the bolometers under the effect of the incident radiation originating from the observed scene and to intrinsic dispersions of the characteristics of bolometers 16. The average continuous level is thus set “in factory” (before putting into service) in the read dynamic range so that the output voltage differences relative to the average continuous level are in the read dynamic range for the widest possible range of observation conditions usually encountered on use of the detector. Usually, the average level is thus not centered in the read dynamic range but rather positioned, for the proposed polarity of the circuit, in the lower third of the dynamic range, to promote the observation of points hotter than the temperature of the ambient background.
Further, as previously mentioned, the compensation bolometers are provided to compensate the common-mode current flowing through the detection bolometers for as wide a focal plane temperature range as possible. In the ideal case where the temperature compensation is perfect, the average continuous level, when the detector observes a scene in the vicinity of the focal plane temperature, thus does not vary along with the temperature and remains ideally positioned in the read dynamic range.
Now, it can be observed that this average level nevertheless varies along with the substrate temperature, and this, by significant proportions. Further, detection bolometers 16 have an intrinsic dispersion of their characteristics, particularly a so-called “offset” dispersion characterized by different detector output voltages in front of a uniform scene, which varies according to the substrate temperature.
Such variations result in a saturation of integrators 28 for a significant proportion of detection bolometers 16 beyond a specific temperature of the substrate. Beyond this temperature, the detector scene dynamic range thus becomes zero. Indeed, scene dynamic range Dsc, which is an essential quantity of the detector, is defined by the amplitude of the temperature variation of any point in a scene, which forms a signal usable at the output, that is, in the electric dynamic range of the detector.
Thus, the previously-indicated phenomena limit the operating temperature range of the detector. To properly understand these limitations, the various involved phenomena have to be developed.
The inventors have observed that the variation of the average continuous level along with the variation, and more particularly the rise, of the substrate temperature, is in particular connected to the phenomenon of “self-heating” of detection bolometers 16 by Joule effect under the effect of the read biasing. Indeed, frame frequency ƒt, that is, the number of full images formed per second, usually is 30, 50, or 60 Hz, which results in a recurrence in the addressing of detection bolometers 16 from 16 to 32 ms. For each recurrence, a Joule power PJ equal at the first order to Vac2/Rac is dissipated in bolometers 16, which causes a heating during each period Tint equal to Tint·PJ/Cth, where Cth is the heat capacity of bolometer 16. The usual quantities of the field result in a heating of bolometer 16 in the order of some ten degrees at the end of each read pulse.
Now, the thermal time constant of bolometers 16 is typically in the range from 8 to 20 ms. As a result, detection bolometers 16 do not have time to cool down and to find a equilibrium temperature close to that of the substrate before being submitted to a new biasing. Thus, the average dynamic equilibrium temperature of the membrane of a bolometer 16 is usually substantially higher by a quantity ΔT of a plurality of degrees than the substrate temperature at which compensation bolometers 24 are maintained. Due to the fact that this difference is proportional to PJ, and thus to 1/Rac, and that resistance Rac of the bolometers decreases along with temperature by approximately 2% per degree for usual thermometric materials with a negative temperature coefficient, the average temperature rise ΔT between detection bolometers 16 and the substrate (compensation bolometers 24) increases along with the substrate temperature.
Accordingly, detection and compensation bolometers 16 and 24 follow resistance curves R(FPT) which progressively move apart when the substrate temperature deviates from the reference temperature.
Finally, quantity (Iac(t)−Icm(t)) which defines the average continuous level after integration over time Tint by the CTIAs, thus regularly increases as the substrate temperature varies beyond the reference temperature. This is the main origin of the displacement of the average continuous level according to the substrate temperature such as shown in curve “state of the art NC” of FIG. 6.
Another source of variation of average continuous level NC with the substrate temperature is linked to the respective nature and geometries of the electronic biasing elements, usually formed of MOS transistors. Indeed, due to the necessary balancing of the voltages available in the two bolometric detection and compensation branches, the voltage across detection bolometer 16 is controlled by an NMOS transistor, and the voltage across compensation bolometer 24 is controlled by a PMOS transistor, or conversely. Such transistors, in addition to being of opposite type, each have specific geometries, particularly in terms of channel width and length, determined as a priority according to contradictory constraints of geometric bulk and electronic noise limitation. As a result, the respective temperature behavior of such transistors, in terms of resulting current respectively in the two bolometric branches, that is, in terms of drift of the average NC according to the substrate temperature, is ordinary far from ideal.
As a result of such phenomena, which have been non-exhaustively discussed, the average NC crosses one or the other limit of the electric output dynamic range of integrator 28, which entails a general saturation of the signal, as soon as the ambient temperature exceeds a few tens of degrees beyond the reference temperature used for the initial pre-setting of the bias voltages. Thereby, the various differential contributions in each branch limit the observable scene dynamic range Dsc, without even considering spatially-dispersive phenomena.
Another limitation has to do with the natural dispersion of the values of the bolometric detection and compensation resistances. Indeed, raw signal Vout in front of a scene at uniform temperature distributes according to a distribution resulting from the natural combined dispersion of the detection and compensation resistances.
For example, a distribution of resistances Rac of detection bolometers 16, though excellent, of +/−1.5% (corresponding to +/−3σ/m), having an average value m compensated up to 90% by a common-mode compensation current itself dispersed at +/−1.5% (corresponding to +/−3σ/m), results in a relative statistic variation on the differential current greater than +/−20% (corresponding to +/−3σ/m). In conditions of setting of the bias currents, of gain of an integrator 28, defined by capacitance Cint 32, and of usual integration duration Tint for this type of circuit, such an input differential current dispersion may easily amount to from 0.3 V to 0.5 V of spatial dispersion ΔNC of the extension of continuous level NC, to be compared with the total electric dynamic range (Del) from 2 to 3 V usually available at the output of integrator 28.
Such a distribution thus already severely limits the useful scene dynamic range, defined as the residual voltage space between the saturation limits of the output (Del), decreased by width ΔNC of the previously-estimated NC distribution, divided by the average response (or responsiveness) =dNC/dTsc of the detector, where Tsc is the scene temperature:Dsc=(Del−ΔNC)/  (1)
When the substrate temperature increases, the relative dispersion of the various bolometric resistances is at best essentially unchanged, admitting that the temperature coefficient TCR of each bolometer is identical, which is only true at the first order. However, all resistances very substantially decrease due to the generally negative TCR, in the order of −2%/K, of usual thermometric materials. Thereby, the absolute dispersion of the bias currents, that is, also of output levels NC, progressively increases with the substrate temperature.
Another limitation is due to natural response dispersions, appearing at the denominator in relation (1), usually of technological origin, of active bolometers 16. Such dispersions also very significantly contribute to limiting the scene dynamic range, all the more as the observed scene is at a high temperature.
Various techniques have been described in prior art to compensate for global variations of the average continuous level or/and offset dispersions.
According to a first technique, for example described in document U.S. Pat. No. 6,433,333, the corrections are obtained by means of a series of factory pre-calibrations at various temperatures.
Another technique, taught by document U.S. Pat. No. 6,690,013, implements a modeled estimate of the corrections to be made according to the substrate temperature.
Another technique further comprises performing a time filtering of the fixed signal components, for example according to document WO 2007/106018.
However, such techniques are based on a correction applied after the forming of the signals by the read circuit. As a result, although they are efficient in terms of stability and dispersion of the corrected signal, they do not address the problem of the progressive scene dynamic range loss when the ambient temperature increases.
To attempt pushing back this limitation, another class of techniques has been developed, with the object of correcting disturbances linked to the substrate temperature upstream of the forming of signals by the read circuits.
Thus, documents U.S. Pat. No. 5,756,999 and U.S. Pat. No. 6,028,309 describe the correction of the offset dispersion of detection bolometers by the application of a variable biasing thereof. Indeed, the signal originating from a detection bolometer directly depends on the current flowing therethrough. By modifying this current, the output average continuous level, and thus the value of its offset, is thus modified. However, this type of correction requires a constraining calibration and implementation protocol, which requires a complex circuit with digital-to-analog converters associated with memories external to the read circuit, to manage, for each frame, the individualized biasing of each bolometer. Further, the in-service modification of the biasings offsets in spatially distributed manner, that is non-uniformly, the operation settings of the suspended membranes with respect to factory calibration conditions, and results in a loss of accuracy, which loss is variable according to the ambient conditions, on gain corrections initially calibrated in factory. This results in an alteration of the image quality.
Another technique intended to compensate average continuous level variations is described in document U.S. Pat. No. 7,417,230. In this document, no common-mode compensation structure is associated with the detection bolometers. Such a technique uses an average output signal Sout of the detection bolometers to dynamically modify by feedback the biasing applied to the detection bolometers. Specific embodiments use bolometers thermalized to the substrate, operating as temperature probes from which a specific independent output signal injected through a feedback loop on the bias voltage(s) of detection bolometers is established. However, this technique first makes up for the absence of thermalized compensation bolometer directly in the signal forming.
Now, an essential limitation connected to the absence of compensation bolometer in the “offset branch” is the need to decrease the bias voltage of detection bolometers when the substrate temperature increases, substantially by the same proportion as the detection bolometer resistance decrease. This mode actually essentially amounts to operating the retina under constant current. However, the signal-to-noise ratio of a bolometer substantially degrades when the bias voltage decreases. As a result, the performance of the detector according to this technique more rapidly degrades with temperature than if the biasing was maintained in the vicinity of its nominal and optimal in-factory setting point. Further, the dynamic principle of the feedback results in eliminating from the useful signal variations due to the average scene temperature. There is thus no more direct way of knowing this temperature, in other words, such a system is not applicable in thermography.
There thus remains a need to substantially correct this scene dynamic range limitation of bolometric detectors which are not temperature-regulated, to have objects capable of operating satisfactorily very far from the reference temperature used for the detector setting.